1. Field of the Invention
This invention relates to a data transmission system or more in particular to a data transmission system suitable for transmitting information under water.
2. Description of the Prior Art
With recent progress in development in the ocean, there is an increased demand for under water transmission including interchange of information between divers and instructions for the operation of an underwater bulldozer or other underwater equipment, to mention a few. Moreover, as the amount of information handled is greatly increasing a demand is growing for quicker and more reliable data transmission.
Generally, data transmission under water uses ultrasonic waves, by the use of which a variety of underwater communication systems have so far been developed. One of such systems employs a method in which the data to be transmitted are encoded and transmitted on respective ultransonic wave signals whose frequencies are different from each other. Therefore, this system requires wave signals of different frequencies, each corresponding to a specific one of the data to be transmitted. Thus, this system has disadvantages such that if twenty-four different data are to be transmitted, as many as twenty-four different frequency signals must be provided. obviate such a disadvantage consists in transmitting the data in a binary code comprising a plurality of bits. The last-mentioned method of transmission is such that a couple of frequency signals f.sub.1 and f.sub.0 corresponding to the binary bits "1" and "0" respectively are transmitted under water at regular intervals of time, which signals are frequency-separated into f.sub.1 and f.sub.0 are decoded at the receiving end.
During transmission under water, however, the ultrasonic wave signals are subject, on the one hand, to interference by waves reflected from the botton or the surface of the water or floating objects, and the received sound reverberates, resulting in a "trailing", on the other.
In order to eliminate the adverse effect of reverberation, it is necessary to transmit an ultrasonic wave of a bit after the reverberation, if any, of the ultransonic wave of the preceding bit has been completely eliminated, thus requiring a considerable period of time for all the data comprising a plurality of bits to be completely transmitted.
Specifically, the time required to completely eliminate the reverberation in the underwater environment should be considered to be approximately 100 msec per bit of information, so that it takes one second to transmit the data of 10 bits as an example.
In addition to the above-mentioned disadvantage resulting in a low data transmission speed, the fact that the time required for data transmission becomes longer the greater the amount of data to be transmitted, the amount of data to be transmitted depending on the number of bits, leads to be shortcoming that the data is more liable to be adversely affected by noise.
Another method of information transmission is a modification of the frequency permutation system, in which a plurality of frequency signals are arranged in different orders at different times and each of the resulting permutations makes up a unit data to be transmitted and received. In the event that the unit data to be transmitted is determined as the numeral 1, 2, 3, ..... or n or the alphabetical character A, B, C, ..... or Z, for example, different patterns depending on the permutations of the frequency signals are made to correspond or are allotted to the responsive data to be transmitted including the numerals 1, 2, 3, ......, n and the alphabetical characters A, B, C, ......, Z in advance. Thus, by sending out the patterns of permutations in a sequence at the transmitting end, it is possible to know the information transmitted as a whole by deciphering the sequential patterns of unit data. Of course, it is necessary to provide a device capable of decoding a variety of arriving unit data immediately at the receiving end.
This data transmission system is generally thought to provide an excellent means of communication under water in view of reduced communication interference due to reverberation and a higher communication speed.
In many data transmission systems, the data introduced in the input or taken out at the output are in the form of binary numbers or binary-coded decimal numbers. Especially in recent years, it is often true that the data are processed in the data processing system before and after transmission and receipt, so that successful commercial application of the above-mentioned data transmission systems requires the provision of a device for converting numerical codes of binary numbers of binary-coded decimal numbers into permutation codes of frequencies at the transmitting end and a converter for converting such permutation codes into numerical codes at the receiving end. Therefore, is improved the data transmission system of this type, especially in the field of underwater communication, by providing the above-described converter means at the transmitting and receiving ends.
The present invention the description of which is made below relates to such converter devices.